The large class of antiproliferative agents includes antimetabolite compounds. A particular subclass of antimetabolites known as antifolates or "antifols" are antagonists of the vitamin folic acid. Typically, antifolates closely resemble the structure of folic acid, including the characteristic p-benzoyl glutamate moiety of folic acid. TS has long been considered an important target enzyme in the design and synthesis of antitumor agents, and a number of folate analogues have been synthesized and studied for their ability to inhibit TS. See, for example, Brixner et al., Folate Analogues as Inhibitors of Thymidylate Synthase, J. Med. Chem. 30, 675 (1987); Jones et al., Quinazoline Antifolates Inhibiting Thymidylate Synthase: Benzoyl Ring Modifications, J. Med. Chem., 29, 468 (1986); Jones et al., Quinazoline Antifolates Inhibiting Thymidylate Synthase: Variation of the Amino Acid, J. Med. Chem., 29, 1114 (1986); and Jones et al., Quinazoline Antifolates Inhibiting Thymidylate Synthase: Variation of the N.sup.10 Substituent, J. Med. Chem. 28, 1468 (1985); and copending U.S. patent application Ser. No. 07/432,338 filed Nov. 6, 1989.